1. Field of the Invention
The present invention relates to a radio base station that communicates with mobile stations using radio waves.
2. Description of the Related Art
In a Personal Handy phone System (hereafter, it will be referred to as PHS), a radio base station that communicates with mobile stations using the TDMA/TDD (Time Division Multiple Access/Time Division Duplex) defined by the PHS standard. There is a demand for enlarging a cover area of each radio station from the aspects of efficient utilization of radio resources and reduction in facility costs.
In order to enlarge the cover area of a radio base station, it is necessary to raise the transmission power and to improve the reception sensitivity so that it is possible to communicate with mobile stations located more distantly. By making the output of a transmission power amplifier higher, it is possible to raise the transmission power of a radio base station. It is, however, necessary to increase the number of antenna elements in order to improve the reception sensitivity. Increasing the number of antenna elements will lead to a problem of having a larger-sized circuit and higher costs. As a solution that makes it possible to equivalently improve the reception sensitivity without increasing the number of antenna elements, the Patent Document 1 discloses a reception diversity method.
The Patent Document 1 discloses a radio base station that communicates with mobile stations using both a TDMA/TDD method and a SDMA (Space Division Multiple Access) method with an array antenna. According to the reception diversity method disclosed therein, in each frame, a link channel from a mobile station is allocated, instead of to a single slot, to a plurality of slots having different timings and different frequencies. The plurality of slots respectively receive signals of the same content, and a reception signal is generated by maximum-ratio combining all the received signals based on corresponding weight information. With this arrangement wherein the same information is received by the plurality of slots having different timings and frequencies, even if one of the slots experiences a reception error, there is a possibility that the same signal from the same mobile station can reach the base station in another slot. Thus, there is a higher possibility of receiving the signal eventually. Consequently, according to the diversity reception method described above, since the reception sensitivity of the radio base station is relatively higher, and it is easier to receive radio waves from a mobile station remotely located, it is possible to enlarge the cover area.
Patent Document 1:
the Japanese Unexamined Patent Application Publication No. 2002-290299.